The present invention relates to a combined heater and heat diffuser for an injection nozzle for the moulding of plastics materials; the invention also relates to a method for manufacturing such a heater-heat diffuser.
Conventionally, an injection nozzle for moulding plastics materials comprises a steel cylindrical tubular core which forms a central longitudinal injection duct for injecting the molten plastics material through one or more injection holes into the moulding cavity of a mould. An electrical resistor is wound around the tubular core for heating the plastics material flowing through the injection duct and for maintaining the parts of the nozzle in contact with the flow of material at a controlled temperature in order to prevent the flow from solidifying. The coils of the resistor are usually closer together in the area near the injection aperture, which is closer to the moulding cavity and thus tends to cool more quickly than the central portion of the nozzle. A capillary thermocouple measures the temperature of the nozzle near the injection aperture. The heat imparted by the resistor tends to accumulate in the central portion of the nozzle, which reaches higher temperatures than the area around the injection aperture which are at times unacceptable for the type of plastics material being moulded, which needs to be kept within a somewhat low temperature range, otherwise the material may deteriorate. It can therefore happen that the resistor is activated as soon as the thermocouple detects that the temperature in the region of the injection aperture has dropped below an established minimum, while the temperature of the central portion of the injection duct, while still acceptable, rises on activation of the resistor until it goes over the maximum admissible value for the material.
Such prior art arrangements mostly use spiral resistors with a rectangular cross section in order to increase the contact surface between the resistor and the tubular core of the nozzle around which it is wound. However, this contact surface constitutes only a fraction of the overall surface of the resistor, so that most of the heat generated by the resistor is not in fact transmitted to the nozzle but is dissipated into the surrounding mould and therefore wasted. In fact the mould needs to be cooled in order to keep the surfaces of the moulding cavity at the lowest possible temperature in order to speed up the solidification of the molten material and thereby shorten moulding cycles.
In order to dissipate heat from the central portion of the nozzle and to diffuse it more evenly along the injection duct, it has been suggested that the resistor be incorporated in a tubular diffuser element made of metal which is fitted on the outside of the tubular core of the nozzle. In this arrangement, a channel-like seat is formed in the outer surface of a cylindrical tubular element for inserting the resistor therein. In this case as well, however, excessive heat is dissipated from the outer surface of the resistor into the surrounding mould; in addition, there is no direct contact between the resistor and the tubular core of the nozzle (and therefore no direct heat transmission by conduction).